The present invention relates to a method and apparatus for removing volatile components from polymer solutions. More particularly, this invention relates to a method for isolating polymers from volatile components within a polymer solution by indirect heat exchange and a unique apparatus which provides such heat exchange.
The removal of volatile components from a polymer solution, often referred to as "devolatilization", is a necessary step in the commercial manufacture of many polymers. In particular, where a polymer is produced from a solution of monomers, it is necessary to remove the solution and unreacted monomers from the final product. For example, about 15% residual monomer and volatiles must be removed from the polymer product in the bulk polymerization of polystyrene. It is noted that polymer solutions containing volatile components may be generated by other procedures as well, such as purification, blending, etc.
A conventional method for isolating polymers from volatile components is by evaporation wherein the polymer solution is heated above the vaporization temperature of the volatile components. There are primarily three classes of processes and apparatuses for removing volatile components from a polymer solution. The apparatus and method utilized is often dependent on the viscosity of the polymer solution. When the viscosity is less than about 10.sup.6 centipoises, a thin film apparatus is used. In these thin film devices, the polymer solution is heated as it descends along the inner surface of a tube or cylinder. Scrapers rotate within the tube to expose new surfaces of the polymer solution. The volatile components evaporate from the solution as it flows down the inner surface. These thin film devices have a high initial cost and are costly to operate. To obtain a high output, very large devices and great expenditures of mechanical energy are required. In addition, these devices impart mechanical shear to the polymer and in some cases may cause deterioration of physical properties.
Where the polymer solution is of a viscosity greater than about 10.sup.6 centipoises, devolatilization may be achieved with the use of vented extruders. These extruders are costly to operate and have a high initial cost per unit of output.
Methods and devices which heat the polymer solution within a zone of indirect heat exchange have been disclosed and are often independent of the viscosity of the polymer solution. However, these processes and devices suffer disadvantages and limitations. Often polymer solutions are subjected to high temperatures for prolonged periods of time. Such exposure causes thermal degradation of heat sensitive thermoplastics such as styrene polymers, including copolymers and mixtures thereof. This thermal degradation often results in discoloration and/or loss of engineering properties, such as impact strength. Those processes and apparatuses which attempt to avoid the degradation of heat sensitive polymers suffer from inefficiency and low output. Typically the polymer is subjected to mild temperatures and a long residence time within the zone of indirect heat exchange. Where this occurs, either output suffers due to a low flow rate through the zone of indirect heat exchange or a very large (and expensive) zone of indirect heat exchange is used to provide the necessary devolatilization.
For example, in U.S. Pat. No. 4,153,501 Fink et al. discloses a method and apparatus for removing vaporizable constituents from melts of thermoplastics by heating the melts of thermoplastics in a defined and gentle manner within a tubular heat exchanger. Fink attempts to avoid degradation of the polymer melts by heating the polymer gradually. This requires a large zone of indirect heat exchange which adds to the initial cost and operating cost of the process.
Other conventional processes which utilize large zones of indirect heat exchange include those of Gemassmer, described in U.S. Pat. No. 3,453,184, and Sessen, described in U.S. Pat. No. 2,853,127. These processes require a long residence time and high heat inputs within the zone of indirect heat exchange, which result in damage to the product in the form of polymer degradation or copolymerization.
It is an object of the present invention to provide a more efficient process and apparatus for removing volatile components from a polymer solution without causing significant damage to the polymer. This is accomplished by reducing the polymer residence time within the zone of indirect heat exchange. Another object of the present invention is to provide a devolatilization apparatus which is efficient and easily manufactured. Other objects will be apparent from the detailed description herein.